ROCK RHEOLOGY AND SHARPNESS OF FOLDS IN SINGLE LAYERS

Many folds in rocks display angular profiles with sharp hinges and str aight limbs. Previous studies by many authors have demonstrated that s uch fold shapes in multilayers result from an intrinsic anisotropy pos sessed by layered or foliated rocks. Our results of two-dimensional fi nite-element modeling show that folds with sharp hinges and straight l imbs may also develop in isolated competent layers under suitable rheo logical conditions. Two basic material properties that affect fold sha pe are non-linearity and anisotropy. Viscous-plastic flow, power-law f low (strain-rate softening), and strain softening are three types of n on-linear behavior. All lead to folds with sharp hinges and straight l imbs. Anisotropy has a similar influence on fold geometry. Angularity of folds in isolated competent layers increases with increasing non-li nearity of the layer or increasing anisotropy of the layer, and a quan titative relationship between fold angularity and degree of non-linear ity or anisotropy may be established. Virtually identical angular fold shapes may be produced by either non-linear or anisotropic layer beha vior. The strain distribution associated with these shapes is very dif ferent, however. For non-linear behavior, strain is focused in the fol d hinges and minimal on the limbs, and for anisotropic behavior the re verse is the case, with shear in the limbs being dominant. These diffe rences suggest that it should be possible to distinguish non-linear fr om anisotropic rheological behavior using layer shape and strain patte rn. Copyright (C) 1996 Elsevier Science Ltd.